postgresql合并string_agg函数的实例

select create_by,string_agg(videoname,',') as videonames from w008_video_addr_info where id in (4248,538,546)
group by create_by

select 
 sa.id,
 info.nickname, 
 (select string_agg(v.videoname,',') 
 from w008_school_assign_video sv 
 join w008_video_addr_info v on sv.videoaddrinfo =v.id 
 where sv.schoolassignment=sa.id and v.is_removed=0 and sv.is_removed=0 
 group by v.is_removed) as videos,
 (select string_agg(to_char(sv.create_date, 'MM-DD HH24:MI'),',') 
 from w008_school_assign_video sv 
 join w008_video_addr_info v on sv.videoaddrinfo =v.id where   
  sv.schoolassignment=sa.id and v.is_removed=0 
 and sv.is_removed=0 group by v.is_removed) as viewtime 
from w008_school_assignment sa 
join w008_user_business_info info on sa.userlongid=info.id where sa.shchoolworkid=2514505674916356

array_agg(expression)

string_agg(expression, delimiter)

create table(empno smallint, ename varchar(20), job varchar(20), mgr smallint, hiredate date, sal bigint, comm bigint, deptno smallint);
insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7499, 'ALLEN', 'SALEMAN', 7698, '2014-11-12', 16000, 300, 30);
insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7499, 'ALLEN', 'SALEMAN', 7698, '2014-11-12', 16000, 300, 30);
insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7654, 'MARTIN', 'SALEMAN', 7698, '2016-09-12', 12000, 1400, 30);
select * from jinbo.employee;
 empno | ename | job | mgr | hiredate | sal | comm | deptno 
-------+--------+---------+------+------------+-------+------+--------
 7499 | ALLEN | SALEMAN | 7698 | 2014-11-12 | 16000 | 300 |  30
 7566 | JONES | MANAGER | 7839 | 2015-12-12 | 32000 | 0 |  20
 7654 | MARTIN | SALEMAN | 7698 | 2016-09-12 | 12000 | 1400 |  30
(3 rows)

select deptno, string_agg(ename, ',') from jinbo.employee group by deptno;
 deptno | string_agg 
--------+--------------
  20 | JONES
  30 | ALLEN,MARTIN

select deptno, array_to_string(array_agg(ename),',') from jinbo.employee group by deptno;
 deptno | array_to_string 
--------+-----------------
  20 | JONES
  30 | ALLEN,MARTIN

select deptno, string_agg(ename, ',' order by ename desc) from jinbo.employee group by deptno;
 deptno | string_agg 
--------+--------------
  20 | JONES
  30 | MARTIN,ALLEN

select deptno, array_agg(ename) from jinbo.employee group by deptno;
 deptno | array_agg 
--------+----------------
  20 | {JONES}
  30 | {ALLEN,MARTIN}

select array_agg(distinct deptno) from jinbo.employee;
array_agg 
-----------
 {20,30}
(1 row)
#不仅可以去重，还可以排序
select array_agg(distinct deptno order by deptno desc) from jinbo.employee;
 array_agg 
-----------
 {30,20}
(1 row)